Household devices, for example, cordless vacuum cleaners, use embedded batteries and external AC/DC power supply/battery charger units. Typically, the input voltage to the battery charger is not regulated and is higher than the battery charging voltage. During charging of the rechargeable battery, the charging current in the battery charger declines as the battery charging voltage increases. However, at full charge, the battery charger continues to supply trickle charging current to the rechargeable battery.
Many household devices use NiCd or NiMH batteries, which can handle trickle charging current up to about C/50 (50 hours charge rate), where a 1 C rate is a charge rate that corresponds to a charging current that would result in a particular rechargeable battery becoming charged in 1 hour. However, due to cost limitations, many battery chargers do not comply with this requirement and overcharge the batteries when left on continuous charge, thus shortening the battery life. This results in significantly less runtime of the household device after only a short period of service.